1. Field of the Invention
The present invention generally relates to a photographing image correction system and, more particularly, to a photographing image correction system for correcting a degraded image on the basis of information on factors which degrade the photographing image and are inherent in a camera body or a lens, which factors include marginal attenuation and distortion.
2. Description of the Related Art
Conventionally, a camera body and a lens have inherent factors which degrade the quality of photographs, such as aberrations and light amount irregularity. For this reason, techniques of eliminating these degradation factors and correcting degraded images have been developed.
For example, a technique of recording camera shake locus data, obtained in a photographic operation of a camera, on a recording medium and restoring an image without blur in a print process in a laboratory on the basis of the camera shake locus data has been proposed and disclosed in U.S. Ser. No. 62,292 (filing date: May 14, 1993), now U.S. Pat. No. 5,365,303 filed by the same assignee as that of the present invention.
In the technique disclosed in U.S. Ser. No. 62,292 (Jpn. Pat. Appln. KOKAI Publication No. 5-323444), however, the amount of data written on the camera body side becomes large, and hence a large space is required for a recording medium. In addition, according to this technique, since information on a degradation in image quality is written in each camera, recording of data is complicated. This tends to cause a camera failure. Furthermore, in the technique, a relatively high cost is imposed on a user.
In general, the brightness of the lens of a camera is determined by an f-number, which indicates the brightness of an image (the center of the image) formed on the optical axis of the lens. The brightness of a marginal portion of the image with respect to the brightness of the center, i.e., the illuminance of a marginal portion of the image, is called marginal illumination.
When the lens is set in a full-aperture state, this marginal illumination decreases owing to the influences of vignetting and the biquadratic law of cosine. As the f-number increases, since the influence of vignetting disappears, the illuminance decreases owing to the influence of the biquadratic law of cosine. This decrease in illuminance will be referred to as marginal attenuation hereinafter.
A photograph having a dark marginal portion is obtained by marginal attenuation of an image in photography.
In general, the photographing lens system of a camera is basically designed to reduce marginal attenuation.
As a technique of preventing this marginal attenuation, for example, Jpn. Pat. Appln. KOKAI Publication No. 63-26079 discloses a technique of correcting the light amount irregularity of an image by image processing.
It is very difficult to design a certain lens system to always prevent marginal attenuation at an arbitrary lens position and an arbitrary f-number. In addition, in order to correct marginal attenuation, a lens system having a complicated arrangement is required. As a result, both the weight and volume of the camera increase.
Furthermore, a lens generally has different magnifications at different portions. For this reason, a distortion is caused depending on the angle of the optical path of a principal ray of an image which is incident on the lens. This distortion includes a positive distortion (pincushion distortion) and a negative distortion (barrel distortion).
The above-mentioned distortion tends to occur in lens groups having an asymmetrical arrangement. In lens groups having a symmetrical arrangement, a distortion caused in the front lens group is canceled by the rear lens group to become less conspicuous. If, however, a distortion occurs, a photographed image does not become a similar figure of an object to be photographed. That is, a straight line of the photographed image is distorted, and the aberration cannot be reduced by increasing the f-number.
As a technique of correcting such a distortion, for example, a technique of correcting a distorted image by image processing is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 62-230267.
This technique uses data compression. According to the technique, an image photographed by a door camera is converted into a digital signal. When this signal is written in a frame memory, the degree of image compression is sequentially changed, thus displaying a corrected image on a CRT.
More specifically, when a negative distortion occurs, the data compression ratio is sequentially increased, in writing each scanning line data, from the left side of the frame to the center line of the frame, and the compression ratio is sequentially decreased from the center line to the right side. Similarly, the compression ratio in the vertical direction is changed such that the compression ratio is sequentially increased toward the center scanning line, and is sequentially decreased toward the upper and lower ends of the frame.
Such data compression is performed with respect to a photographed image to correct the distorted image into a proper image.
In general, however, instead of using the above-described correction technique, the photographing lens system of a camera is designed to reduce distortion by itself.
Even if the distortion is to be reduced by a photographing lens system itself, since a lens position is limited in terms of structure in many cases, it is very difficult to design the lens system so as to always prevent a distortion at an arbitrary f-number. In addition, in order to correct distortions, a lens system having a complicated arrangement is required. As a result, both the weight and volume of the camera increase.
Furthermore, the correction technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 62-230267 is suitable for a case wherein an image is photographed and corrected, as an analog or digital image signal, and the corrected image is displayed on a display screen, as in a case of a television camera or a television set. However, in a camera using a film, since an image is optically recorded, the image cannot be optically corrected by using the above correction technique.